Standard DIN EN 590 stipulates the minimum requirements for the characteristic values and properties of diesel fuels. Table 1 shows an overview of the important characteristic values for fuel.
TABLE 1ProductDiesel (sulphur-free)[logo]NameDiesel fuelHolborn EuropaStandardEN 590 (February 2000)Raffinerie GmbHLast updated onFebruary 2002SpecificationsHolborn item no.DieselDIN EN 590ParameterUnitminmaxminmaxTesting procedureAppearanceclear, free of sediments andVisualsoluble acid fractions (TAN)Color2ISO 2049Density at 15° C.kg/m3820845820845EN ISO 12185Flash point n.P.M.° C.59>55EN 22719Sulphur contentmg/kg1010DIN EN ISO 14596and DIN 51400 T11Viscosity at 40° C.mm2/s2.04.52.04.5EN ISO 3104Boiling pattern until 250° C.Vol %<65<65EN ISO 3405Boiling pattern until 350° C.Vol %8585EN ISO 340595 vol. % point° C.360360EN ISO 3405Cetane number CFR test engine51.051.0EN ISO 5165Cetane number BASF test engine52.252.2DIN 51773Cetane index4646EN ISO 4264Total dirt accumulationmg/kg2424EN 12662Neutralization numbermgKOH/g0.20.2DIN 51 558Con. carbon. V. 10% dest. residueWeight %0.30.3EN ISO 10370Copper corrosion (3 h 50° C.)Corr. degree11EN ISO 2160Electrical conductivitypS/m50DIN 51 412-2Oxidation stabilityg/m32525EN ISO 12205Cloud pointSummer product° C.5EN 23015Transition product° C.−3Winter product1° C.see driveabilitylimitCFPP2Summer product° C.−20EN 116Transition product° C.−13−10Winter product1° C.see driveability−20limitAsh contentWeight %0.010.01EN ISO 6245Water contentmg/kg200200EN ISO 12937Oiliness (HFRR WS 1.4)μm460460ISO 12156-1Polyaromatic compoundsWeight %11.011.0IP 391/95Filtering capacitymin2SEDAB TestDelivery timesSummer product04/15-09/1404/15-09/30Transition product (autumn)09/15-10/3110/01-11/15Winter product111/01-02/2811/16-02/28Transition product (spring)03/01-04/1403/01-04/14Driveability limit3Cloud point° C.−5−6−7−8−9DIN EN 23015CFPP° C.−30−28−25−23−22DIN EN 116Remarks:Sampling done according to DIN 51750 T1 + T21premium diesel (11/01-02/28) with at least 150 mg/L WASA/L diesel2to a maximum of 500 mg MDFI/kg diesel3defined through the combination of cloud point after short sedimentation test and CFPP
Especially important in all of this is ignitability, which is described with the cetane number or CN. Briefly explained, one can say that the ignitability of a diesel fuel has an important impact on the engine's combustion process, as well as on noise and emissions. In principle, the higher the cetane number, the shorter the time elapsed between fuel injection and start of combustion (ignition delay). Consequently, the combustion noise decreases as the pressure increase speed decreases. Maximum pressures and temperatures also become lower, something that has a positive effect on nitrogen oxide emissions. In a cold start, a higher cetane number has a favorable impact on HC emissions.
The cetane index given in the standard is alternatively calculated from density and boiling range and is only partially correlated with the CN numbers obtained from the engine because the behavior of ignition accelerators is not taken into account. The CN numbers are determined empirically in special test engines. The compression ratio in the CFR engine and the air intake in the BASF engine can be varied, so that the fuel being tested can be changed.
The objective is to compare the ignitability of the fuel being tested with fuels with known cetane numbers and, if need be, to determine the cetane number through interpolation. In the standard, cetane (n-hexadecane) was arbitrarily assigned the cetane number of 100 and alpha-methyl naphthalene was assigned the cetane number of 0. By mixing the components, one can produce a fuel that will have the same ignitability as the fuel to be tested. The cetane number sought will then correspond to the volumetric share of cetane in the fuel mixture.
To determine the ignitability of a fuel, the test engines are operated in accordance with Table 2.
TABLE 2Dimensions and measuring conditions of the BASF test diesel and the CFR test dieselDescriptionBASF test dieselCFR test dieselConstructionTechnical testing stand of theCooperative FuelBadische Anilin- & Soda FabrikResearch Committee ofAGthe American Society ofAutomotive EngineersManufacturerHermann Ruf, MannheimWaukesha Motor Co.,Waukesha, Wisconsin,USABoringmm90S 2.6Strokemm120114.3Displacementcm3850613CompressionFixed 18.5:1AdjustableVolumetric control  6-21:1Mode of operationSwirl chamberSwirl chamberMeasuring procedure and measuring conditionsProcedureConst. ignition delayConst. ignition delayRPMrpm1000 ± 10 900 ± 9 Intake air temp.° C.20 ± 566 ± 1Coolant temp.° C.100 ± 2 100 ± 2 Oil temperature° C.70 ± 557 ± 8Injection start°CA b. UDP2013Ignition start°CA b. UDP00Injected quantitycm3/min  8 ± 0.5  13 ± 0.2Ignition delay displayElectronic ignition delay meterIgnition delay meterStandardDIN 51 773ASTM-D 613 62 TMeasuring rangeCaZ30-10030-1001Manufacturer: Hermann Ruf Co., 68 Mannheim-Neckarau2Manufacturer: Waukesha Motor Co., Waukesha, Wisconsin, USA
In the CFR engine, the injection point has been fixed to 13° crankshaft angle before upper dead point (° CA b. UDP). The compression ratio is varied in such a way that combustion always starts in the UDP, i.e. with a 13° CA ignition delay. A sensor measuring the cylinder pressure determines the start of combustion. In this case, it is assumed that the start of combustion can be exactly determined with this sensor and the analog processing of the signal. Tests have shown, however, that the curve of the cylinder pressure is only of limited use for determining the start of combustion. In this context, FIG. 1 shows the pressure curves of diesel engines with different combustion processes. According to the principle, the DI engine has the highest pressure increase speed and even the start of combustion which is in the 4-2° CA crankshaft angle before UDP range can still be determined relatively precisely from the pressure. In the swirl chamber engine, however, the determination of the start of combustion becomes significantly harder because of the already slower energy turnover. This problem also applies especially to the test engine when used in a standardized way, as it is executed as a swirl chamber engine too.
Compared to different test engines, the standard gives the accuracy of the process in the 2.8-4.8 CN range. The repeat accuracy lies between 0.8 and 1 CN. Operation is manual and lasts 20-30 minutes per cetane number.
Tests were (and still are being) performed to determine the cetane number with another instrument (especially with vegetable oils) outside of the engine. Thus, the Ignition Quality Tester (IQT) of the Advanced Engine Technology Co. of Ottawa, Ontario, and the Fuel Ignition tester of the Fueltech AS Co, of Trondheim, Norway, are used mostly in Canada and the USA. Both measuring instruments determine the ignitability along the measured ignition delay of the fuel in a constant volume, heated high-pressure chamber. Automobile manufacturers are skeptical about the standardization of these processes that take place outside the engine. A fundamental improvement of the engine process has not been found.
The following problem areas have been detected in assessing the standard processes for determining the cetane number:                The accuracy of the process can be improved upon.        The process is time-consuming; it cannot be automated and allows no online display.        In the process, the start of combustion (ignition delay) is determined from the cylinder pressure after analog processing and displayed directly. No exact calculation of the start of combustion takes place.        An evaluation of the cetane numbers of vegetable oils with the needed accuracy has so far been impossible.        The injection moment is set mechanically. There is no operational check with a needle stroke display.        The combustion process in the swirl chamber is no longer contemporary.        
It is, therefore, the task of the invention to suggest a process that will make a fast and reliable characterization of the ignitability of fuels possible.